Two tiny doc things, combine regression fix

[appgurueu]

Fixes #14312 (last commit), among two doc improvements.

How to test

Observe that the regression has regressed. Read the docs and commit messages.

[grorp]

The fix for the regression works.

The two documentation improvements could be squashed into one "Minor documentation fixes" commit.

[sfan5]

Replaced the fix with my own commit as discussed on IRC.

[appgurueu]

I haven't tested the last commit well yet, but the logic should check out, and stuff (like signs) still looks right.

[sfan5]

I will trust your testing

[appgurueu]

Tested. Code:

minetest.after(1, function()
	local E = minetest.formspec_escape
	local t = {"size[4,4]"}
	for x = 0, 1 do
		for y = 0, 1 do
			local tmod = ("[combine:2x2:0,0=rgbv.png:%d,%d=rgbv.png"):format(2*x-1, 2*y-1)
			table.insert(t, ("image[%d,%d;1,1;%s]"):format(x, y, E(tmod)))
		end
	end
	minetest.show_formspec("singleplayer", "dev:test", table.concat(t))
end)

rgbv.png